U.S. Pat. No. 6,289,965 B1 discloses a conventional window blind which includes a head rail, a base rail, an expandable window covering between the head rail and the base rail, and a spring motor. The spring motor includes a frame, a drive drum, an idler gear, a take-up drum, a coil spring. The drive drum is rotatably mounted to the frame. The idler gear is rotatably mounted to the frame and is operably connected to the drive drum. Rotation of the idler gear causes rotation of the drive drum. The take-up drum is rotatably mounted on and concentric with the idler gear. The idler gear is rotatable independently of the take-up drum. The coil spring is interconnected between the take-up drum and the drive drum. The coil spring is biased into a wound orientation on the take-up drum. When a user pulls the base rail downwardly to displace the base rail such that the coil spring is unwound from the take-up drum and is wound on the drive drum, a friction force is generated between the coil spring and the take-up drum. Since the coil spring is normally made of metal, the take-up drum is likely to become worn due to friction with the coil spring. Thus, the conventional window blind may have a relatively short service life.